A wide variety of cardiac pacemakers are known and commercially available. Pacemakers are generally characterized by which chambers of the heart they are capable of electrically sensing, the chambers to which they deliver pacing stimuli, and their ability to respond, in their operation, to indicia of normal or abnormal cardiac functioning. Some pacemakers deliver pacing stimuli at fixed, regular intervals without regard to naturally occurring cardiac activity. More commonly, however, pacemakers sense electrical cardiac activity in one or both of the chambers of the heart, and inhibit or trigger delivery of pacing stimuli to the heart based on the occurrence and recognition of sensed intrinsic electrical events. A so-called "VVI" pacemaker, for example, senses electrical cardiac activity in the ventricle of the patient's heart, and delivers pacing stimuli to the ventricle only in the absence of electrical signals indicative of natural ventricular contractions. A "DDD" pacemaker, on the other hand, senses electrical signals in both the atrium and ventricle of the patient's heart, and delivers atrial pacing stimuli in the absence of signals indicative of natural atrial contractions, and ventricular pacing stimuli in the absence of signals indicative of natural ventricular contractions. The delivery of each pacing stimulus by a DDD pacemaker is synchronized with prior sensed or paced events.
Pacemakers are also known which respond to other types of physiologically-based signals, such as signals from sensors for measuring the pressure inside the patient's ventricle or for measuring the level of the patient's physical activity. In recent years, pacemakers which measure the metabolic demand for oxygen and vary the pacing rate in response thereto have become available. Perhaps the most popularly employed method for measuring the need for oxygenated blood is to measure the physical activity of the patient by means of a piezoelectric transducer. Such a pacemaker is disclosed in U.S. Pat. No. 4,485,813 issued to Anderson et al.
In typical prior art rate-responsive pacemakers, the pacing rate is determined according to the output from an activity sensor. The pacing rate is variable between a predetermined maximum and minimum level, which may be selectable by a physician from among a plurality of programmable upper and lower rate limit settings. When the activity sensor output indicates that the patient's activity level has increased, the pacing rate is increased from the programmed lower rate by an incremental amount which is determined as a function of the output of the activity sensor. That is, the rate-responsive or "target" pacing rate in a rate-responsive pacemaker is determined as follows:
Target Rate=Programmed Lower Rate+f(sensor output) where f is typically a linear or monotonic function of the sensor output. As long as patient activity continues to be indicated, the pacing rate is periodically increased by incremental amounts calculated according to the above formula, until the programmed upper rate limit is reached. When patient activity ceases, the pacing rate is gradually reduced, until the programmed lower rate limit is reached.
In an effort to minimize patient problems and to prolong or extend the useful life of an implanted pacemaker, it has become common practice to provide numerous programmable parameters in order to permit the physician to select and/or periodically adjust the desired parameters or to match or optimize the pacing system to the patient's physiologic requirements. The physician may adjust the output energy settings to maximize pacemaker battery longevity while ensuring an adequate patient safety margin. Additionally, the physician may adjust the sensing threshold to ensure adequate sensing of intrinsic depolarization of cardiac tissue, while preventing oversensing of unwanted events such as myopotential interference or electromagnetic interference (EMI). Also, programmable parameters are typically required to enable and to optimize a pacemaker rate response function. For example, Medtronic, Inc.'s Legend and Activitrax series of pacemakers are multiprogrammable, rate-responsive pacemakers having the following programmable parameters: pacing mode, sensitivity, refractory period, pulse amplitude, pulse width, lower and upper rate limits, rate response gain, and activity threshold.
For any of the known rate-responsive pacemakers, it is clearly desirable that the sensor output correlate to as high a degree as possible with the actual metabolic and physiologic needs of the patient, so that the resulting rate-responsive pacing rate may be adjusted to appropriate levels. A piezoelectric activity sensor can only be used to indirectly determine the metabolic need. The physical activity sensed can be influenced by upper body motion. Therefore, an exercise that involves arm motion may provide signals that are inappropriately greater than the metabolic need. Conversely, exercises that stimulate the lower body only, such as bicycle riding, may provide a low indication of metabolic need while the actual requirement is very high. Therefore, it would be desirable to implement a rate-responsive pacemaker that is based on a parameter that is correlated directly to metabolic need.
Minute ventilation (V.sub.c) has been demonstrated clinically to be a parameter that correlates directly to the actual metabolic and physiologic needs of the patient. Minute ventilation is defined by the equation: EQU V.sub.c =RR.times.VT
where RR=respiration rate in breaths per minute (bpm), and VT=tidal volume in liters. Clinically, the measurement of V.sub.c is performed by having the patient breathe directly into a device that measures the exchange of air and computes the total volume per minute. The direct measurement of V.sub.c is not possible with an implanted device. However, V.sub.c can indirectly measured by monitoring impedance changes in the patient's thoracic cavity. Such impedance measurements can be performed with implantable circuitry and implantable leads.
In general, the measurement of the impedance present between two or more sensing locations is referred to as rheography. Typically, rheographic measurement involves delivering a constant current pulse between two "source" electrodes, such that the current is conducted through some region of a patient's tissue, and then measuring the voltage differential between two "recording" electrodes to ascertain the impedance of the tissue, the voltage differential arising from the conduction of the current pulse through the tissue.
A pacemaker with rheographic capabilities can measure thoracic impedance in a patient by delivering a known current between two of the pacemaker's electrodes. A pacemaker capable of measuring thoracic impedance with rheography is disclosed in U.S. Pat. No. 4,702,253 issued to Nappholz et al. on Oct. 27, 1987; the Nappholz et al. patent is hereby incorporated by reference herein in its entirety. In the Nappholz arrangement, the magnitude of the change of the impedance signal corresponds to the tidal volume and the frequency of change corresponds to respiration rate.
In U.S. Pat. No. 4,721,110 issued to Lampadius on Jan. 26, 1988, there is described a rheographic arrangement for a cardiac pacemaker in which the base pacing rate of the pacemaker is determined, in part, by a rheographically-obtained respiration rate signal. According to the Lampadius patent, rheographic current pulses can interfere with the sensing of electrical cardiac signals, and are therefore delivered only during the refractory period immediately preceding delivery of a stimulating pulse.
There continues to be a wide interest among implantable pulse generator manufacturers in using rheographic techniques to measure such physiological parameters as minute ventilation. This is due in part to the fact that rheographic techniques allow measurement of physiological parameters without the need for a special sensor (such as the piezoelectric sensor on the canister of activity-sensing pacemakers). Rheographic techniques require, at most, the use of multiple electrodes located on a standard pacing lead.
There are currently several commercially available implantable devices which employ rheographic techniques to adjust the pacing rate according to metabolic needs. For example, the Biorate device manufactured by Biotec International, Bologna Italy, uses a bipolar (i.e., two-electrode configuration) rheographic arrangement to monitor the patient's respiratory rate. The Meta-MV device manufactured by Telectronics, Inc., Englewood, Colo., uses a tripolar rheographic arrangement to monitor the patient's respiratory rate. The Precept device manufactured by CPI, St. Paul, Minn., uses a tetrapolar rheographic arrangement to monitor the patient's pre-ejection interval (PEI), stroke volume (SV), and heart tissue contractility. It is well known that other manufacturers are exploring or have explored the use of rheographic techniques to monitor physiological parameters.
The parameters measured by rheography in presently known devices are essentially derived from two sources: respiration and cardiac systole. Respiration is typically measured with either a bipolar configuration in which the pacemaker's conductive canister and a large surface area electrode are used, or with a tripolar configuration using standard pacing/sensing leads having much smaller surface area electrodes. With configurations using small surface area leads, the source and recording functions must be kept spaced apart, to avoid detection of contact resistance variations. Signals representative of respiratory rate, tidal volume, and minute ventilation can be derived using bipolar or tripolar configurations.
Detection of cardiac systole, on the other hand, is currently believed to require a tetrapolar configuration. Pre-ejection interval, stroke volume, and contractility can be derived from a tetrapolar rheographic configuration.
Attempts have been made and described in the literature to utilize bipolar rheographic configurations (e.g., Tip-to-Can or Tip-to-Ring sensing configurations), with standard pacing/sensing leads, to detect both respiration and systole. However, such methods do not appear to lead to a useful result, since only multipolar techniques achieve consistent impedance measurement using small surface area electrodes.
It has been the inventors' experience that bipolar rheographic configurations, which derive a signal that is a composite of cardiac systole effects and respiration effects, are highly susceptible to variations in the contact resistance of the electrodes. The contact resistance varies because respiration and cardiac systole induce a deformation on the pacing/sensing lead, thus changing the pressure of the tip electrode on the myocardial tissue. Screw-in leads reduce, but do not eliminate, the inconsistency in bipolar rheographic measurement.
Thus, it is believed that different electrode configurations are required to detect both respiration and cardiac systole. That is, both the position and the function of the electrodes must be different depending upon whether cardiac systole or respiration is to be measured.
A further consideration in the selection of an electrode configuration for a pacemaker is the necessity of accurate sensing of cardiac electrical signals, including those associated with atrial contractions (i.e., "P-waves"), and those associated with ventricular contractions ("R-waves" or "QRS complexes"). Atrial sensing is especially difficult, due to the low magnitude of P-waves in relation to R-waves and the resulting problems of discriminating P-waves from R-waves. With only a single atrial electrode, the atrial signal (P-wave) detected by the atrial electrode is very similar to the ventricular signal (R-wave) appearing at that same electrode. Mere amplification of the atrial signal, therefore, does not assist in discriminating atrial from ventricular signals detected in the atrium, or in discriminating atrial signals from myopotentials or external interference.
The use of more than one atrial electrode has proven to be useful in atrial signal discrimination. This is because two atrial electrodes placed at different locations in the atrium will detect similar ventricular signals but atrial signals that are different in both amplitude and morphology.
In conventional pacemaker systems, the electrode configuration is fixed, such that a specific function is assigned to each electrode (the electrode placement, of course, also being fixed). For example, in the above-noted Telectronics Meta-MV device, the tip electrode of the bipolar pacing/sensing lead is always "recording" (i.e., receiving current), the ring electrode of the lead is always the "source" (i.e., producing a current); and the conductive pacemaker canister is always a source and recording electrode. This configuration allows measurement of impedance variation primarily due to respiration.
In the CPI Precept device, the tip electrode is always the "source" the canister is also always the "source" and two ring electrodes on the pacing lead proximal to the tip electrode, are always "recording". This configuration allows measurement of impedance variation primarily due to cardiac systole.
There has yet to be shown in the art, however, an implantable device capable of dynamically adapting its electrode configuration for the purposes of obtaining different physiological measurements.